Printing devices such as inkjet printers may comprise one or more printheads for depositing ink onto a print medium. These printing devices may be used in a wide variety of applications, and may include computer printers, plotters, copiers, and facsimile machines. Often a printhead forms part of a removable printer pen or cartridge. In certain printing devices it may be desired to apply a treatment fluid, as well as ink, to a print medium. For example, a pre-treatment fluid may be “underprinted”, i.e. deposited prior to depositing ink on a print medium, and/or a post-treatment fluid may be “overprinted”, i.e. deposited after depositing ink on a print medium. The use of treatment fluid may improve one or more characteristics of a printed output, for example improve at least one of ink adhesion, durability, and ink absorption.